Natural Convection Heat Transfer of Suspension

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Natural convection heat transfer of suspension in a rectangular vessel was investigated. A vertical wall of the vessel was heated and the opposite vertical wall was cooled. SiO_2 fine particles with 3 or 6 μm of mean diameters were dispersed into water or Dimethyl Sulfoxide (DMSO) aqueous solution. The particle size distributions were narrow. The refractive index of the DMSO solution was matched to that of the SiO_2 particles and the velocity of the natural convection was measured with Particle Image Velocimetry. By using the water-SiO_2 suspension, the temperature and particle concentration distributions in the suspension and the average heat transfer coefficient on the vertical wall were measured under various conditions of the temperature difference, the initial particle concentration, the mean diameter of particle, and the width of the vessel. The mean Nusselt number of the natural convection of the suspension was compared with that of water. Sedimentation of particles affected the natural convection of suspension. When the initial particle concentration was large and the temperature difference was small, the natural convection was depressed and the Nusselt number became small. As the width of the vessel became wider, the Nusselt number decreased. When the initial particle concentration was small and the temperature difference was large, convection cells were stratified and the natural convection in the cells was active and the Nusselt number was more than 80% of that of water. If the number of the convection cells was two, the Nusselt number had the maximum when the position of the interface between the convection cells was near half of the vessel height.